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渣漿泵抽送懸浮物的特性曲線
一、泵抽送懸浮物時的性能變化
當(dāng)懸浮物液流在流道內(nèi)的流動速度超過那個假定的雷諾數(shù)Re具有確定值的速度值時，流動變?yōu)樽阅Ａ鲃?。在這種情況下，抽送懸浮物和清水時的水力摩擦損失是相同的。
當(dāng)懸浮物流動時，葉輪流道內(nèi)水力摩擦損失變化將影響泵內(nèi)總的水力損失。在具有強烈混合液流即具有混合損失的壓水室內(nèi)，水力損失為自模損失。
懸浮物在葉輪流道內(nèi)的流動速度主要與圓周速度有關(guān)，在渣漿泵中這種速度相當(dāng)大。
應(yīng)該考慮從偽層流狀態(tài)過渡到自模狀態(tài)，在假定的雷諾數(shù)Re很寬變化范圖內(nèi)發(fā)生。如果偽層流狀態(tài)在Re' <3000時開始，那么自模狀態(tài)在Re=11000-12000時穩(wěn)定。在靠近最佳狀態(tài)，以及一般雷諾數(shù)大于對應(yīng)自模狀態(tài)值的大多數(shù)情況下大流量狀態(tài)，和只有在小流址狀態(tài)，在泵一些斷面上可能產(chǎn)生與自模狀態(tài)有顯著區(qū)別的流動狀態(tài)。
這樣，泵的大多數(shù)狀態(tài)將是自模狀態(tài)。因此泵過流部件流道內(nèi)的損失，無論在抽送清水或者抽送懸浮物時將是相同的。
當(dāng)很小流量時，流速下降，以至葉輪流道內(nèi)流動狀態(tài)從自模轉(zhuǎn)變?yōu)槲蓜踊蛘邔恿鳡顟B(tài)，在這種情況下，水力損失遠(yuǎn)比同樣速速時自模狀態(tài)下?lián)p失大(參閱圖2-3- 4).
過流部件流道內(nèi)水力摩擦損失的增大，將導(dǎo)致小流量時泵揚程特性曲線的“凹陷”現(xiàn)象發(fā)生(圖3-6-2).在很小流量時，葉輪和壓水室之間大量流體進行強烈交換，這就導(dǎo)致紊動或者自模狀態(tài)發(fā)生，楊程略有增加，接近泵抽送為質(zhì)液體時的揚程。
懸浮物密度越大，對應(yīng)自模狀態(tài)開始的雷諾數(shù)Re就越大(在相同懸浮物流速時)，因此，當(dāng)懸浮物密度增大時，揚程特性曲線在小流量的情況下降低較為明顯。
因為泵抽送清水和懸浮物時理論揚程相同，所以它們的水力功率也與懸浮物和清水的密度有關(guān)。軸承和填料函摩擦損失功率，占水力功率的百分?jǐn)?shù)不大。泵抽送懸浮物時圓盤損失功率與抽送清水時圓盤損失功率的比值遠(yuǎn)大于懸浮物和清水的密度之比。其理由說明如下。在葉輪腔內(nèi)大部分液體的角速度，一次近似時采用等于葉輪角速度的一半。隨著腔內(nèi)液體到泵軸的距離減小，圓周速度降低。當(dāng)懸浮物在腔內(nèi)圓周速度明顯減小時，流動狀態(tài)不再是自模狀態(tài)。這種現(xiàn)象與所研究兩個腔的范圍摩擦增大有關(guān)。這樣，在泵抽送懸浮物時，觀察到圓盤摩擦損失增大。這種效應(yīng)在液體側(cè)向吸入的泵上特別顯著，在泵上葉輪后蓋是整體的(無穿軸孔)，即存在具有很小圓周速度的很大表面。因此，在這個區(qū)域內(nèi)懸浮物以很小速度旋轉(zhuǎn)。
根據(jù)IO. H莫吉列夫斯基進行的磁鐵粉和硅鐵懸浮物高液度試驗資料，為了考慮圓周摩擦附加損失，必須將泵的功率比泵抽送同樣密度均質(zhì)液體時功率增大10%~12%。因此，如果知道泵抽送清水時的功率No,那么抽送懸浮物時的功率為
現(xiàn)在我們研究泵抽送懸浮物時汽蝕余R相對于抽送清水時汽蝕余量的變化。
因為泵在很寬流量范圍內(nèi)揚程特性曲線(很小流量狀態(tài)除外)，與抽送請時一樣，所以，各種損失，特別是葉輪入口損失也與抽送清水時一樣。
現(xiàn)有試驗資料證明，與抽送清水泵相比，抽送懸浮物泵的汽蝕特性曲線明顯惡化(圖8-6-3）泵抽送懸浮物時允許汽蝕余量Oh比抽送清水時汽蝕余量有所增加，其理由說明如下。
物體繞流時，在一定距離上可以觀察到流動減速。例如，圓柱繞流時，均勻速度場在圓柱前面5~6倍半徑的距離上開始破壞。經(jīng)常出現(xiàn)物體繞流分流奇點，其速度等于零。
我們研究在相對運動中葉片入口邊繞流。因為抽送磨蝕性固液混合物泵葉輪葉片相對厚度很大，葉片入口段繞流可以看作圓柱表面繞流。在葉輪入口可以觀察到懸浮物液流的紊流狀態(tài)。由于在葉片入口邊之前液流減速，液流沿著一些流線的速度降低到臨界速度u，在此速度時，液流開始具有不完全破壞結(jié)構(gòu)。因為葉片入口邊繞流的分流點(在此點相對速度等于零)處在葉片前面，在這個區(qū)域內(nèi)形成黏塑性流體(圖3-6-4)，這種流體已不是牛頓流體。這種流體與塑性體相似，以接近臨界速度v的速度運動。
由于在葉片入口處整個液流發(fā)生排擠，這就導(dǎo)致在不是塑性體所占據(jù)的液流其余部分內(nèi)速度增大，其中包括葉片背面上的速度(圖3-5-4,點a).因此，在這點速度最大，而壓力最小。因為懸浮物液流局部速度比清水(相同流量時)的局部速度增長快，所以泵抽送懸浮物時汽蝕特性曲線惡化。
在葉片前面點b區(qū)域內(nèi)，由于速度降低，就形成壓力很高的區(qū)域。壓力增量用Ap表示。作用在體積為V.制動部分的懸浮物上的沖量與spf成正比(式中，f為體積V顆粒最大截面面積).在入口邊之前壓力局部下降，與速度頭和葉輪入口相對速度w1有關(guān)，可以認(rèn)為sp=Cpwf/2 (式中，C為比例系數(shù))。
根據(jù)動量變化方程式可以認(rèn)為，
制動顆粒體積與液體流量成正比，即與葉輪入口處的是速度c1成正比，因此，形成塑性體最大截面面積越大，在葉輪入口液流附加排擠就越大。渣漿泵廠家
Characteristic curve of pumping suspended solids
CHANGES IN PERFORMANCE OF PUMPING SUSPENDED MATERIALS
When the velocity of suspended solids flow in the channel exceeds that assumed Reynolds number Re with a fixed value, the flow becomes self-mode flow. In this case, the hydraulic friction loss is the same when pumping suspended solids and clean water.
When the suspension flow moves, the change of hydraulic friction loss in impeller passage will affect the total hydraulic loss in the pump. In a pressurized water chamber with strong mixing fluid flow, i.e. mixing loss, the hydraulic loss is self-model loss.
The velocity of suspended solids in impeller passage is mainly related to the circumferential velocity, which is quite large in slurry pump.
Consideration should be given to the transition from pseudo-laminar state to modelling state, which occurs in the hypothetical Reynolds number wide variation paradigm. If the pseudolaminar flow begins at Re'< 3000, the self-model state is stable at Re=11000-12000. In most cases when the Reynolds number is greater than the corresponding self-model state value, the flow state with large flow rate may be significantly different from the self-model state on some sections of the pump in the condition of small flow location.
In this way, most of the state of the pump will be self-model state. Therefore, the loss in the passage of the flow passage of the pump components will be the same whether it is pumping clean water or suspended solids.
When the flow rate is very small, the flow velocity decreases, so that the flow state in the impeller passage changes from a model to a turbulent or laminar flow state. In this case, the hydraulic loss is much greater than that in the self-model state at the same speed (see Figure 2-3-4).
The increase of hydraulic friction loss in flow passage of flow passage components will lead to the "depression" phenomenon of pump head characteristic curve at small flow rate (Figure 3-6-2). At very small flow rate, a large amount of fluid is exchanged intensively between impeller and pressure chamber, which leads to turbulence or self-modelling, and the Yang-Cheng increases slightly, approaching pumping as liquid mass. Lift of body time.
The Reynolds number Re at the beginning of self-model state increases with the increase of suspended matter density (at the same suspended material flow rate). Therefore, when the suspended matter density increases, the head characteristic curve decreases obviously under the condition of small flow rate.
Because the theoretical lift of pumping clear water and suspended solids is the same, their hydraulic power is also related to the density of suspended solids and suspended solids. The friction loss power of bearings and stuffing box accounts for a small percentage of hydraulic power. The ratio of disc loss power to disc loss power in pumping suspended solids is much larger than that in pumping clear water. The reasons are as follows. In the first approximation, the angular velocity of most liquid in the impeller chamber is equal to half of the angular velocity of the impeller. The circumferential velocity decreases as the distance between the liquid in the cavity and the pump shaft decreases. When the circumferential velocity of suspended solids in the cavity decreases significantly, the flow state is no longer self-modeled. This phenomenon is related to the increase of friction in the range of the two cavities studied. In this way, when the suspended matter is pumped by the pump, it is observed that the friction loss of the disc increases. This effect is particularly significant in lateral liquid suction pumps, where the impeller back cover is integral (without perforation), i.e. there is a large surface with a small circumferential velocity. Therefore, the suspended matter rotates at a very small speed in this area.
According to IO. H. Mogilevski's high liquid test data of magnet powder and ferrosilicon suspension, in order to consider the additional loss of circumferential friction, the power of the pump must be increased by 10%~12% compared with that of pumping homogeneous liquid with the same density. Therefore, if the power of pumping clear water is known to be No, then the power of pumping suspended solids is zero.
Now we study the change of cavitation residual R in pumping suspended solids relative to that in pumping clean water.
Because the pump head characteristic curve in a wide flow range (except for very small flow state) is the same as when pumping, so all kinds of losses, especially the impeller inlet loss, are the same as when pumping clean water.
The available test data prove that the cavitation characteristic curve of pumping suspended solids pump is worse than that of pumping clean water pump (Figure 8-6-3). The allowable cavitation allowance Oh of pumping suspended solids is higher than that of pumping clean water. The reasons are as follows.
The deceleration of flow can be observed at a certain distance when an object flows around it. For example, when a cylinder flows around it, the uniform velocity field begins to destroy at a distance of 5 to 6 times the radius in front of the cylinder. Singularities of flow diversion around objects often occur, and their velocities are equal to zero.
We study the flow around the blade inlet in relative motion. Because the relative thickness of impeller blade of pumping abrasive solid-liquid mixture pump is very large, the flow around the blade inlet can be regarded as the flow around the cylinder surface. Turbulence of suspended solids can be observed at the inlet of impeller. Because the liquid flow decelerates before the blade inlet, the velocity of the liquid flow along some streamlines decreases to the critical velocity U. At this velocity, the liquid flow begins to have an incomplete destruction structure. Because the shunt point (where the relative velocity is equal to zero) of the flow around the blade inlet is in front of the blade, viscoplastic fluid is formed in this area (Fig. 3-6-4), which is no longer Newtonian fluid. This fluid is similar to the plastic body and moves at a speed close to the critical velocity v.
Because of the squeezing of the whole flow at the blade entrance, the velocity in the rest of the flow, which is not occupied by the plastic body, increases, including the velocity on the back of the blade (Fig. 3-5-4, point a). Therefore, the velocity is the highest at this point and the pressure is the lowest. Because the local velocity of suspended solids flow increases faster than that of clear water (at the same flow rate), the cavitation characteristic curve deteriorates when suspended solids are pumped.
In front of the blade
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